Ok, after a bit of research, the source files are in the openhab-webui project, here
With the help of ImageMagick convert command line tool, I was able to create a png32 version of all the PNG files, and then I placed them in the following folder:
$OPENHAB_CONF/icons/myplatform
With this, I can get the desired content with the iconset argument added to the URL like this:
http://server:8080/icon/temperature?state=25.8&format=png&iconset=myplatform
So clearly, even if I could not identify it, there is code in openHab that looks in subfolders of $OPENHAB_CONF/icons named as the given iconset name when it searches for a given image.
This is much cleaner than creating the converted files, adding a prefix/suffix and placing them all in the classic folder as this way, they live in a clearly identified folder.